Sound knowledge about the quantitative requirements for the nutritionally indispensable amino acids in adult human protein nutrition is fundamental to the assessment of diet adequacy and planning of diets for population groups. The goals of this research are to firmly establish the requirements for these nutrients, driven by the hypothesis that earlier national and international recommendations for the amino acid requirements of adults were far too low. Previous work on this grant has supported this hypothesis. In consequence, the earlier estimates are now undergoing substantial, upward revision at the international and national levels and in a major way dependent on the work that we have carried out during this grant period. There remain significant, unresolved major issues and these require investigation before a profile of the indispensable amino acid requirement for healthy adults world-wide can be stated with some confidence. Hence, we will continue a series of stable isotope-labeled amino acid tracer studies, to close the gap in present knowledge, in healthy subjects from two geographic regions (US-Boston; India-Bangalore) and in undernourished subjects (Bangalore) to assess the general applicability of the requirement estimates in healthy populations and to better understand the consequences of under nutrition on the utilization of and requirements for these indispensable amino acids. The specific aims of this research are: 1) to complete and evaluate ongoing studies on the requirements for the sulfur amino acids, methionine (with and without cystine); 2) to explore the impact of different methionine/cysteine intakes on glutathione homeostasis, methionine cycle activity and methyl group metabolism. This aim will be conducted in both healthy US and Indian subjects and undernourished Indian adults; 3) to further establish the requirements for the total aromatic amino acids from phenylalanine alone; 4) to explore the impact of dietary tyrosine on the requirement for phenylaIanine alone; 5) to confirm the predictions of the requirements for valine as derived from the requirements for leucine; 6) to examine the consequences of adaptation to a low lysine and low threonine diet on the post-prandial utilization (PPU) of a protein meal limiting in either lysine or threonine. The long-term objective of these studies to improve the quantitative knowledge about human nutrient requirements and factors that affect them. [unreadable] [unreadable]